Blade-like shaft of a hip-joint prosthesis

ABSTRACT

Blade-like stem ( 10 ) of a hip joint prosthesis for anchoring in the femur, having a portion ( 19 ) comprising a prosthesis neck ( 18 ) on the one hand and a femur-anchoring portion ( 21 ) tapering towards a distal end ( 20 ) on the other hand, the lateral narrow side ( 22 ) of which comprises a distal straight portion ( 23 ) and a proximal arcuate portion ( 11 ), the straight portion ( 23 ) extending over a length (L D ) of from 60% to 75% of the total length (L G ) of the stem ( 10 ).

The invention relates to a blade-like stem of a hip joint prosthesis for anchoring in the femur, having a portion comprising a prosthesis neck on the one hand and a femur-anchoring portion tapering towards a distal end on the other hand.

Such blade-like stems are generally known. Reference is made in this respect merely by way of example to EP 0 240 815 B1. A stem corresponding to FIG. 6 is shown and described therein. Accordingly, that stem 1 comprises a portion comprising a prosthesis neck 7 on the one hand and a femur-anchoring portion 2 tapering towards a distal end 3 on the other hand. That femur-anchoring portion widens conically all round from the distal end 3 in the direction of the stem longitudinal axis 4. The medial narrow side 5 merges out of the said cone into a continuously curved arc which ends in a plane which, running perpendicular to the prosthesis neck axis 6, terminates the prosthesis neck 7 towards the stem blade, i.e. the femur-anchoring portion 2. The prosthesis neck 7 ends in an outwardly conically tapering pin on which a spherical joint head (not shown) can be placed.

The lateral narrow side 8 widens out of the conical widened portion to form a trochanter wing 9 before merging, via a shoulder of the stem blade or anchoring portion, into the said prosthesis neck termination plane.

The described stem is used for cementless anchoring in the femur. In principle, however, the present invention is intended to relate also to cemented stems.

In both cases, a stem-receiving space, that is to say a corresponding cavity, has to be provided beforehand in the femur, that being effected by means of a shaping instrument, especially a rasp, corresponding to the shape of the stem. Such shaping instruments or rasps correspond exactly to the geometry of the stem in question or differ specifically therefrom in order to obtain a predetermined undersize for a press-fit or a predetermined oversize as space for a cement mantle.

Once the hip joint has been opened and the neck of the femur resectioned, in the proximal femur the bony bearing is prepared for receiving the anchoring stem. According to the shape of the stem, the bony anchoring bed is created using a suitable shaping instrument, especially a rasp, by movement down along the stem axis. For moving down into the medullary space, which is filled with spongy bone and soft tissue, the rasp is driven forwards by means of a weight acting as a hammer or using some other suitable instrument. In the case of a curved stem axis, the shaping instrument or rasp is moved down in an arc along a curved path, while in the case of a straight stem axis the rasp is driven forward along a straight line corresponding substantially to the axis of the proximal medullary space.

For cementless anchoring of hip stems, the configuration of the anchoring portion as a straight stem has proved especially suitable clinically. This concept is distinguished by a secure implantation technique, high primary stability and good ingrowth behaviour. The surgical technique for such stems requires the medullary space to be opened not only in the plane of the resection surface of the neck of the femur, but also further laterally into the region of the greater trochanter. Reference is made in this respect to FIG. 4. That Figure shows that a resection of portions of the tendon insertions in that region is also necessary. The extent of that resection of course depends upon the individual shape of the proximal femur and upon the shaping of the straight stem.

More recently, there has been an increase in implantations of joint endoprostheses carried out using minimally invasive surgical techniques. The aim of such techniques is more rapid rehabilitation of the patient, which is associated with a reduction in pain and a shorter stay in hospital. Minimally invasive surgical techniques keep operative trauma, especially in respect of the functionally significant structures, to a very low level. For the functioning of the hip joint the important structural features are the muscles and tendons. The aim of minimally invasive implantation techniques is inter alia to avoid resections and detachments of tendon and muscle insertions in the region of the greater trochanter. Classic straight stems accordingly have disadvantages for the use of minimally invasive techniques.

To avoid resections in the region of the tendon insertions on the greater trochanter, in the case of straight stems the lateral area can be chamfered in the region of the trochanter. Straight stems having a flattened shoulder area are known in principle. In this respect, reference is made merely by way of example to the so-called Müller straight stem shown and described in “Technique d'implantation de prothèses totales de Müller par voie latérale transglutéale”, Encyclopédie Medico-Chirurgicale (Paris) 44666, 1991.

The aim of that flattened shoulder is to avoid major defects in the region of the ridge of the trochanter. When shaping that flattened portion, a proportion of the lateral stem area, which proportion is constant within the size system, was generally configured with straight shaping inclined relative to the stem axis or with a radius. The rasp corresponding to the implant was generally made geometrically identical to the implant. From the technical standpoint, the rasp is used to create an undercut in the region of the greater trochanter, as can be seen in FIG. 5.

In the case of implantation of a hip stem, the bony bed is shaped using rasps of increasing size up to the size giving the best fit, the rasp in question following the shape of the existing bed formed by the preceding rasp size. Because the distal portion of the straight stem is effected by moving down along a straight axis, compromises are made in terms of exact fit in respect of the inclined or curved shoulder area. That is influenced, however, by the surgeon's rasp technique and individual bone quality.

Starting from the above-mentioned prior art, the aim of the present invention is to provide a blade-like stem of the kind mentioned at the beginning that is especially suitable for minimally invasive surgical techniques. In so doing, the advantages of conventional straight stem implants should be retained, but muscle and tendon insertions should be protected as much as possible.

That problem is solved according to the invention as follows: the lateral narrow side of the stem comprises a distal straight portion and a proximal arcuate portion, the straight portion extending over a length of from 60% to 75% of the total length of the stem.

The shape of the convex proximal arcuate portion is particularly important, an especially preferred form being distinguished by the fact that the arcuate portion is in the form of a “tractrix” which is described or defined by the proximal end of the lateral narrow side of the stem on introduction thereof (or of a corresponding rasp) into a complementary cavity in the femur while the contact between the lateral-distal and proximal-medial stem contour on the one hand and the associated boundary of the cavity on the other hand is maintained. The optimum shape of the proximal-lateral shoulder area is therefore derived from the guidance of the stem in the bony bed, as shown in FIG. 2. The lateral-proximal stem curvature 11 of the stem 10 corresponds to the curve 12 which is described by the proximal end of the lateral narrow side of the stem 10 on introduction thereof into a complementary cavity in the femur, on condition that the contact between the lateral-distal stem contour 13 and the proximal-medial stem contour 14 on the one hand and the associated boundary of the cavity (not shown herein) on the other hand is retained. A stem 10 constructed in accordance with the invention is otherwise shown in side view (ventral or dorsal), and moreover in comparison with a conventional blade-like stem according to FIG. 6 or EP 0 240 815 B1. FIG. 1 shows very clearly what measures have been taken in comparison with the prior art. The lateral side of the trochanter wing 9 according to FIG. 6 has been trimmed as a result of the lateral-proximal arcuate portion 11 with the advantage that there is correspondingly less interference with the ridge of the trochanter and, in particular, muscle and tendon insertions are also less severely affected during implantation or formation of the cavity for the stem 10. The contact zones in the region of the so-called Shenton's arch (region 14 in FIG. 2) and at the lateral-distal end (region 13 in FIG. 2) describe an arc along the lateral shoulder. That arc is described by a polynomial (curve of the xth order). That curve can follow on continuously from the lateral-distal stem geometry; preferably, however, it forms an intersection therewith. In any case, the lateral-proximal arcuate portion is so configured that over the entire introduction path of the distal straight stem portion the shoulder has exact contact with or constant spacing from the bone structure in the trochanter region. There is thus achieved an optimum, exactly fitting shoulder area with respect to the bony bed with gap-free seating or with an exactly predetermined gap for cement, according to whether a cementless or cemented implantation is to be carried out.

In principle, it would also be possible to continue the lateral-distal straight portion in the proximal region in accordance with FIG. 7. That embodiment, however, is very much less protective of muscle and tendons than the embodiment according to the invention in accordance with FIG. 1. That can be seen very clearly especially from FIG. 3, which shows the introduction paths of the proximal end of the lateral narrow side of the stem for the versions in accordance with FIG. 6, FIG. 7 and FIG. 1. The introduction path 15 applies to the construction in accordance with FIG. 6 (prior art). The introduction path 16 applies to the construction according to FIG. 7 and the introduction path 17 applies to the configuration according to the invention in accordance with FIG. 1. Accordingly, there is least interference with the trochanter in the case of the embodiment according to the invention.

Also with reference to FIG. 1, it should be pointed out that the Figure shows a blade-like stem 10 of a hip joint prosthesis for anchoring in the femur, the stem being constructed in accordance with the invention. The stem has a portion 19 comprising a prosthesis neck 18 on the one hand and a femur-anchoring portion 21 tapering towards a distal end 20 on the other hand, the lateral narrow side 22 of which comprises a distal straight portion 23 and a proximal arcuate portion 11, the straight portion 23 extending over a length L_(D) of from 60% to 75% of the total length L_(G) of the stem 10. In the embodiment shown, the lateral straight portion 23 can merge continuously into the lateral arcuate portion 11, that is to say it is tangential. As already mentioned, however, it is entirely acceptable for that transition to comprise a discontinuity, that is to say to be obtuse-angled.

As already mentioned above, it is especially advantageous for the lateral arcuate portion 11 to be in the form of a kind of “tractrix” which is described or defined by the proximal end of the lateral narrow side of the stem 10 on introduction thereof into a complementary cavity in the femur while the contact between the lateral-distal and proximal-medial stem contour on the one hand and the associated boundary of the cavity on the other hand is maintained. In this respect reference is again made to FIG. 2.

Depending upon the size of the stem and the external conditions, the proximal arcuate portion 11 is preferably configured with a constant, however especially continuously or discontinuously changing radius of between 200 mm and 500 mm.

It has also proved practical for the proximal arcuate portion 11 to be configured with a radius that becomes increasingly smaller continuously or discontinuously from distal to proximal.

The arcuate portion 11 can especially also be in the form of a hyperbolic, parabolic or elliptical portion such that towards the distal the portion in question merges into the conical straight portion 23 at a predetermined point at which the tangent forms with the stem axis 24 an angle equal to half the cone angle.

In connection with the stem itself, it should also be mentioned that the anchoring portion widens conically over the length of the lateral-distal straight portion 23 starting from the distal end 20 in the direction of its longitudinal axis 24 either all round or only laterally-medially.

It should also be mentioned that the cross-section of the stem 10 is preferably rectangular, but may also be trapezoidal or rhombic.

The mentioned all-round conical widening of the anchoring portion 21 has a cone angle of about from 0.5° to 6°, especially about from 1° to 3°, and particularly also on the ventral and/or dorsal side.

All the features disclosed in the application documents are claimed as being important to the invention, insofar as they are novel over the prior art individually or in combination.

REFERENCE NUMERALS Prior Art:

-   1 stem -   2 femur-anchoring portion -   3 distal end -   4 longitudinal axis -   5 medial narrow side -   6 prosthesis neck axis -   7 prosthesis neck -   8 lateral narrow side -   9 trochanter wing

Invention:

-   10 stem -   11 proximal-lateral stem curvature (lateral-proximal arcuate     portion) -   12 curve -   13 lateral-distal stem contour -   14 proximal-medial stem contour -   15 introduction curve for stem according to FIG. 6 -   16 introduction curve for stem according to FIG. 7 -   17 introduction curve for stem according to FIG. 1 -   18 prosthesis neck -   19 portion -   20 distal end -   21 femur-anchoring portion -   22 lateral narrow side -   23 distal-lateral straight portion -   24 stem axis 

1-8. (canceled)
 9. A hip joint prosthesis stem for anchoring in the femur, wherein the stem comprises a blade-like shape and comprises: a prosthesis neck portion; and a femur-anchoring portion attached to the prosthesis neck portion and tapering towards a distal end, the femur-anchoring portion comprising a lateral narrow side comprising a distal straight portion extending over a length of from 60% to 75% of the total length of said stem, the lateral narrow side further comprising a proximal arcuate portion.
 10. The prosthesis of claim 9, wherein said distal straight portion merges into said proximal arcuate portion in a manner selected from the group consisting of continuously and with a discontinuity.
 11. The prosthesis of claim 9, wherein said proximal arcuate portion comprises a shape corresponding to a curve described by a proximal end of the lateral narrow side of the stem on introduction thereof into a complementary cavity in the femur while contact between a lateral-distal and proximal-medial stem contour and an associated boundary of the cavity is maintained.
 12. The prosthesis of claim 9, wherein said proximal arcuate portion comprises a shape corresponding to a curve described by a proximal end of the lateral narrow side of a corresponding rasp on introduction thereof into a complementary cavity in the femur while contact between a lateral-distal and proximal-medial rasp contour and an associated boundary of the cavity is maintained.
 13. The prosthesis of claim 9, wherein said proximal arcuate portion comprises a radius of curvature of between 200 mm and 500 mm selected from the group consisting of a constant radius of curvature, a continuously changing radius of curvature, and a discontinuously changing radius of curvature.
 14. The prosthesis of claim 9, wherein said proximal arcuate portion comprises a radius of curvature that becomes increasingly larger in a manner selected from the group consisting of continuously and discontinuously from a distal end to a proximal end of the proximal arcuate portion.
 15. The prosthesis of claim 9, wherein said anchoring portion widens conically over the lateral-distal straight portion starting from the distal end toward the proximal end.
 16. The prosthesis of claim 15, wherein said anchoring portion widens about its entire circumference.
 17. The prosthesis of claim 15, wherein said anchoring portion widens only in the lateral-medial direction.
 18. The prosthesis of claim 9, wherein the cross-section of said stem is selected from the group consisting of rectangular, trapezoidal and rhombic.
 19. The prosthesis of claim 15, wherein said anchoring portion widens conically with a cone angle selected from the group consisting of about from 0.5° to 6°, and about from 1° to 3°.
 20. The prosthesis of claim 19, wherein said cone angle is defined only on the ventral side of the anchoring portion.
 21. The prosthesis of claim 19, wherein said cone angle is defined only on the dorsal side of the anchoring portion.
 22. A hip joint prosthesis stem for anchoring in the femur, wherein the stem comprises a blade-like shape and comprises: a prosthesis neck portion; and a femur-anchoring portion attached to the prosthesis neck portion and tapering towards a distal end, the femur-anchoring portion comprising a lateral narrow side comprising a distal straight portion and a proximal arcuate portion, wherein said proximal arcuate portion comprises a shape corresponding to a curve defined by a proximal end of the lateral narrow side of the stem on introduction thereof into a complementary cavity in the femur while contact between a lateral-distal and proximal-medial stem contour and an associated boundary of the cavity is maintained.
 23. The prosthesis of claim 22, wherein said distal straight portion merges into said proximal arcuate portion in a manner selected from the group consisting of continuously and with a discontinuity.
 24. A method of constructing a hip joint prosthesis with at least a proximal-lateral portion, a distal-lateral portion, and a proximal-medial portion, said method comprising: choosing a hip joint prosthesis shaft with a determined distal-lateral portion and proximal-medial portion; obtaining a curve defined by the proximal-lateral portion of the prosthesis shaft upon introduction of the shaft into a cavity in a femur such that contact between the distal-lateral and proximal-medial portions of the shaft and the associated boundary of the cavity will be maintained; and defining a proximal-lateral portion of said prosthesis shaft to generally fit said curve.
 25. The method of claim 24 wherein the proximal-lateral portion of the shaft exactly fits the curve. 